This invention relates to an idling rpm feedback control method for internal combustion engines, and more particularly to a method of this kind which is adapted to discriminate between a decelerating condition of an engine and an idling rpm feedback controlling condition of the engine to prevent engine stall and enhance the driveability of the engine.
In an internal combustion engine, the engine can easily stall due to a drop in the engine speed when the engine is operated in an idling condition at a low temperature of the engine cooling water or when the engine is heavily loaded with loads by head lamps, air conditioner, etc. in a vehicle equipped with the engine. To eliminate such disadvantage, an idling rpm feedback control method has been propsoed e.g. by Japanese Patent Provisional Publication (Kokai) No. 55-98628, which comprises setting desired idling rpm in dependence upon engine load of the engine, detecting the difference between actual engine rpm and the desired idling rpm, and supplying a quantity of supplementary air to the engine in a manner responsive to the detected difference so as to minimize the same difference, to thereby control the engine rpm to the desired idling rpm.
According to this proposed method, if the idling rpm feedback control is carried out immediately when the engine is decelerated toward the desired idling rpm region and before the engine speed drops to the desired idling rpm region, the resulting controlled quantity of supplementary air is much smaller than a required quantity to be supplied to the engine, because on such occasion the supplementary air quantity is controlled to such a small value as to immediately bring the engine rpm to the desired idling rpm. If on such occasion the clutch of the engine is disengaged, there occurs a sudden drop in the engine speed, which can cause engine stall.
To avoid this disadvantage, an idling rpm feedback control method has been proposed e.g. by Japanese Patent Provisional Publication (Kokai) No. 55-98629. According to this proposed method, in transition from a decelerating condition to an idling rpm feedback controlling condition, the quantity of supplementary air is controlled in decelerating mode wherein supplementary air is previously supplied to the engine in a quantity corresponding to a drop in the engine speed before the idling rpm feedback control is started, to thereby ensure smooth transition to the idling operation.
However, once the idling rpm feedback control has been started, it is desirable for prompt control of the engine rpm to continue the feedback control even when the engine is released from electrical loads such as head lamps to have its speed increased to such high engine rpm that the supplementary air quantity should be controlled in decelerating mode. To this end, it is necessary to exactly determine whether the engine is in an operating condition requiring control of the supplementary air quantity in decelerating with the throttle valve fully closed mode or in an operating condition requring feedback control of the same quantity.